Overscan
The picture area beyond the "safe" visible frame — the edge a TV normally crops, but which demo coders fought to draw into.
Home-computer video chips drew a plain single-colour border around the image by default, meant to hide the ragged edges of the sweep. The Amiga had explicit overscan modes that let a program claim the whole area; on the C64 it took "opening the borders" — carefully timed writes to the VIC-II that tricked the chip into not drawing the border at all.
The point was always the same: an effect filling the entire screen impressed more than one boxed inside a frame. Opening the edges became a matter of course in any serious demo.